1. Field of the Invention
The present invention relates to a pneumatic tire structured such that a rolling resistance can be reduced while maintaining steering stability.
2. Description of the Related Art
Under a condition that concern for global environment protection is raised in recent years, a rolling resistance of a tire greatly contributes to fuel consumption performance of a vehicle, and it is necessary to effectively reduce it. Conventionally, there has been proposed a technique of reducing the rolling resistance by changing a composition of tread rubber, however, since it affects the steering stability and wear resistance performance of the tire not a little, there is strongly desired a method which can reduce the rolling resistance independently from the rubber composition.
In order to reduce the rolling resistance, it is important to suppress an energy loss when the tire rolls, and it has been known that a strain generated in a tread portion is greatly involved in such an energy loss. Further, after the present inventor has devoted himself to study, it has been known that a strain tends to be locally concentrated in a region 20 in the periphery of a groove bottom of a main groove 23 facing a shoulder block 25 as shown in FIG. 5 when the tire comes into contact with the ground, and an increase of the strain causes an energy loss so as to deteriorate the rolling resistance.
Japanese Unexamined Patent Publication No. 2001-1719 describes a pneumatic tire in which a narrow groove extending continuously in a tire circumferential direction is provided in a surface of a shoulder block in an outer side in a tire width direction than a ground-contact end. However, since this tire only intends to reduce a road noise by absorbing a vibration by the narrow groove when the tire rolls, and a reduction of rigidity in the tire circumferential direction is caused not a little by the narrow groove, there is a risk that the steering stability is deteriorated. Further, this document defines maintenance of the fuel consumption performance as an effect obtained by such a matter that it is not necessary to make rubber of a tread portion thick.